Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net revenues	$ 159,298	$ 164,110	$ 166,280	$ 167,921	$ 151,207	$ 130,626	$ 126,769	$ 129,473	$ 657,609	$ 538,075	$ 448,059
Operating income	31,147	32,624	34,710	27,277	23,698	22,636	23,371	18,472	125,757	88,177	77,650
Net income	21,280	23,326	25,582	19,995	18,914	16,923	18,977	14,595	90,182	69,409	62,804
Limited partners' interest in net income	$ 14,215	$ 16,690	$ 19,370	$ 14,365	$ 13,891	$ 12,178	$ 14,425	$ 10,486	$ 64,639	$ 50,980	$ 50,611
Common Units
Net income per limited partner unit:
Common (basic) (in usd per unit)	$ 0.58	$ 0.68	$ 0.79	$ 0.59	$ 0.57	$ 0.50	$ 0.59	$ 0.43	$ 2.65	$ 2.09	$ 2.08
Common units - (diluted) (in usd per unit)	$ 0.58	$ 0.68	$ 0.79	$ 0.59	$ 0.57	$ 0.50	$ 0.59	$ 0.43	$ 2.65	$ 2.09	$ 2.07

Net income per limited partner unit: [Abstract]

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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Aggregate amount of net income allocated to limited partners.

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Per unit of ownership amount after tax of income (loss) available to limited partnership (LP) unit-holder and units that would have been outstanding assuming the issuance of limited partner units for dilutive potential units outstanding.

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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